This is why I ferment 5.5 gallons batches in a 6.5 gallons carboy (which really could probably hold 7 gallons to the neck). For larger volume and larger OG brews I will use a blow off if necessary, but even with 6 gallons of 1.104 barleywine I didn't need it. I also agree with MrClean, start at the lower limit of the temperature limit and raise it slowly once fermentation starts.
Before I bought the big carboy I had a 1.070 Robust Porter dump probably half a gallon into my fermentation fridge. The blow off plugged just like yours, shot the top off of it, the entire fridge was covered by porter inside. Turned out to be one of my best beers! This was last November, still have two bottles and it won second place porter in a local competition back in October.